JPH03195963A - Electrochemical gas sensor element - Google Patents

Abstract

PURPOSE:To exhibit a stable performance by adding a solid electrolyte in a substrate composed of a porous material, providing a plurality of electrodes on the same surface of the substrate, providing a solid electrolyte layer while covering the electrodes and spaces therebetween and bringing part of the substrate into contact with water. CONSTITUTION:A working electrode 20 and a counter electrode 30 both com posed of platinum and a reference electrode 40 composed of gold are formed on the surface of a substrate 10 by a sputtering method. A 30 mum thick solid electrolyte layer 50 is formed by casting a perfluorosulfonate polymer solution over the electrodes 20, 30 and 40. The resultant substrate 10 is fixedly mounted on a supporter 60 made of an acrylic resin and assembled so that one end of the substrate 10 is soaked in water 80 in a water reservoir 70. The substrate 10 sucks up the water 80 in the water reservoir 70 at one end and keeps a solid electrolyte in the substrate 10 at a constant moisture content. At the same time, the substrate supplies the inside of the solid electrolyte layer 50 with water via a contact portion between the substrate 10 and the solid electro lyte layer 50 to keep also the moisture content of the electrolyte layer 50 con stant.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an electrochemical gas sensor element, and more particularly, to a gas sensor element that utilizes an electrochemical reaction to exhibit a function of detecting gas, etc. in the atmosphere. It is something.

[Conventional technology]

The basic structure of a gas sensor element that utilizes an electrochemical reaction is to connect a plurality of electrodes with an ion conductor, that is, an electrolyte, to cause an electrochemical reaction.

Conventionally, liquid electrolytes or gel electrolytes have been used as materials for ion conductors, but these have had the problem of poor device durability and reliability due to liquid leakage and solvent evaporation. In order to solve these problems, progress has been made in the development of gas sensor elements using inorganic or organic solid electrolytes.

Examples of the inorganic solid electrolyte include β-alumina, Nasicon, Risicon, and stabilized zirconia. However, solid electrolytes made of these inorganic substances have high impedance at room temperature, and therefore are in a state where it is difficult for ions to conduct thereat at room temperature. Therefore, in general, the gas sensor element is heated to have a low impedance before use, but this means that the power consumption of the gas sensor element increases, which is not practical.

Examples of organic solid electrolytes include polymers belonging to cation exchange resins such as polystyrene sulfonate, polyvinyl sulfonate, perfluorosulfonate polymer, and perfluorocarboxylate polymer. Among these resins, perfluorosulfonate polymers are widely used as the most practical ones, and for example, there is one called Nafion (trade name, manufactured by DuPont).

The reason why the perfluorosulfonate polymer is preferable is that it has a large degree of cationic dissociation, that is, has a small impedance, or is relatively stable thermally and electrochemically. Further, since the perfluorosulfonate polymer is soluble in a solvent, a solid electrolyte layer made of the perfluorosulfonate polymer can be easily formed on an insulating substrate or an electrode by casting a solution. This means that the gas sensor element is easy to manufacture.

[Problem to be solved by the invention]

However, it is known that the physical properties of perfluorosulfonate polymers as described above vary greatly depending on humidity. Since physical property values such as impedance and gas permeability of the perfluorosulfonate polymer have a large influence on the sensitivity of the gas sensor, the humidity dependence of the physical property values directly appears as the humidity dependence of the sensitivity. It is undesirable to correct the humidity dependence of the sensitivity using an electric circuit that processes the detection signal because it is costly, and it is desirable to improve the humidity dependence of the physical properties of perfluorosulfonate polymers. There is.

The humidity dependence of the physical properties of perfluorosulfonate polymers is due to changes in the water content of the perfluorosulfonate polymers as the humidity changes. In other words, the higher the water content, the lower the impedance and the better the gas permeability.
It is used as a solid electrolyte for gas sensors. Therefore, if the perfluorosulfonate polymer is always kept in a high water content state, the above-mentioned physical properties can be maintained as a solid electrolyte for gas sensors, and the humidity dependence of the sensitivity of the gas sensor element can be eliminated. become.

Such humidity-dependent problems similarly occur with solid electrolytes other than the above-mentioned perfluorosulfonate polymer.

Therefore, the objective of this invention is to take advantage of the advantages of solid electrolytes such as perfluorosulfonate polymers while suppressing the humidity dependence of the physical properties of these solid electrolytes, so that sensitivity can be less dependent on humidity and exhibit stable performance. An object of the present invention is to provide an electrochemical gas sensor.

[Means to solve the problem]

The electrochemical gas sensor element of the present invention, which solves the above problems, includes a solid electrolyte inside a substrate made of a porous body, a plurality of electrodes are provided on the same surface of the substrate, and the solid electrolyte covers each type and between them. A layer is provided and used in such a way that a portion of the substrate is in contact with water.

As the substrate, porous bodies in a broad sense are used, including so-called porous materials and fibrous materials having minute pores or gaps among various inorganic and organic materials, such as cellulose filters and glass cloths. That is, in the present invention, as will be described later, any material can be used as long as it has pores or gaps that have water absorbency to suck water through capillary action and gas permeability to the extent that gas molecules can pass through and reach the electrode. It's good. The substrate contains a solid electrolyte to be described later. The solid electrolyte may be contained in the substrate in the form of a solution dissolved in a suitable solvent.

Three types of electrodes, a working electrode, a counter electrode, and a reference electrode made of platinum, gold, etc., are usually arranged side by side on the same surface of the substrate, but the specific structures and arrangements of each type are based on the usual electrochemical formula. Something similar to a gas sensor is freely available.

Materials for the solid electrolyte included in the substrate and the solid electrolyte layer covering the electrodes include the aforementioned polystyrene sulfonate, polyvinyl sulfonate, perfluorosulfonate polymer, perfluorocarboxylate polymer, etc.
The same material as that of the solid electrolyte layer in a normal electrochemical gas sensor is used.

The solid electrolyte layer can cover the electrodes and between them, and its neck shape, thickness, etc. can be arbitrarily set as long as it can conduct ions well. In this invention, the gas component to be detected passes through the porous substrate and reaches the electrode, so the solid electrolyte layer only needs to serve as an ion conductor in the electrochemical reaction. Therefore, unlike conventional electrochemical gas sensors, it is not necessary to form a thin solid electrolyte layer to improve gas permeability. It is preferable to form an electrolyte layer. Specifically, it varies depending on the type of solid electrolyte layer, but about 3
It is preferable to form it to a thickness of 0 pm or more. When the solid electrolyte layer has a thickness of 30 pm or more, gas components hardly pass through the solid electrolyte layer, and most of the gas components reach the electrodes from the porous substrate side.

In order to bring a portion of the substrate into contact with water, a water reservoir may be provided adjacent to the substrate. The water tank is made of various resins, ceramics, glass, etc., and is formed into a container shape that can store the amount of water necessary to keep the water content of the solid electrolyte constant. The water tank is provided at a position adjacent to the substrate, and is arranged so that a part of the substrate comes into contact with water contained inside the water tank. Water holding means other than a water tank may be provided.

[For production]

When a portion of the substrate made of a water absorbing material is in contact with water, water is drawn into the substrate by capillary action and spreads over the entire substrate. When the moisture contained in the solid electrolyte layer evaporates due to changes in environmental humidity, etc., the moisture absorbed by the substrate is supplied to the solid electrolyte layer from the contact area between the substrate and the solid electrolyte layer. Moisture content remains constant. Since the substrate is successively replenished with water, the water content of the solid electrolyte layer is maintained at a constant level over a long period of time, and the solid electrolyte layer can be maintained in a high water content state through ionic conduction or the like.

Since the porous substrate contains the solid electrolyte, gas components can easily pass through the pores and gaps of the porous body from the surface of the substrate, diffuse into the solid electrolyte inside, and reach the electrodes. Therefore, unlike conventional electrochemical gas sensors, there is no need for gas components to reach the electrodes by passing through a solid electrolyte layer covering the electrodes. Therefore, since the solid electrolyte layer only needs to perform ion conduction, it becomes possible to increase the thickness of the solid electrolyte layer and thereby perform ion conduction favorably.

To explain the above operation in detail, the electrochemical gas sensor is capable of detecting gas components by causing an electrochemical reaction between the gas components at the interface between the working electrode and the electrolyte. Therefore, in order to increase the sensitivity of the gas sensor, it is necessary to allow more gas components to reach the interface between the working electrode and the electrolyte.To achieve this, the thickness of the solid electrolyte covering the working electrode must be reduced as much as possible. The thinner the electrode, the better.However, since the detection current flows only after the ions generated by the electrochemical reaction at the interface between the working electrode and the electrolyte are conducted to the counter electrode, the ion conduction between the working electrode and the counter electrode must also be good. In order to improve ion conduction in a solid electrolyte, the thickness of the solid electrolyte must be increased.

That is, as mentioned above, in order to improve the gas components reaching the working electrode, that is, to improve the gas permeability, the thickness of the solid electrolyte must be made thin, and in order to improve the ion conduction from the working electrode to the counter electrode. There are mutually contradictory demands that the thickness of the solid electrolyte must be increased.

However, in this invention, the substrate made of a porous body containing a solid electrolyte has the effect of allowing gas components to pass through well and reach the electrodes, and the solid electrolyte layer only needs to perform ion conduction. This makes it possible to form the layer sufficiently thick to provide good ion conduction.

〔Example〕

Next, embodiments of the present invention will be described in detail below with reference to the drawings.

As shown in FIGS. 1 and 2, three electrodes, a working electrode 20, a counter electrode 30, and a reference electrode 40, made of platinum, gold, or other electrode materials are placed on the surface of a substrate 10 made of a porous material such as a cellulose filter. is formed. As a method for forming the electrode, a conventional metal thin film forming method such as plating or vapor deposition is employed. A solid electrolyte layer 5° made of perfluorosulfonate polymer or the like is formed to cover the various parts 20 and between them. One end of each type 20 is extended and exposed to the outside of the solid electrolyte layer 50, and serves as a terminal portion 22, 32, 42 for connection to an external circuit.

The substrate IO includes a solid electrolyte similar to the material of the solid electrolyte layer 50. However, the amount of solid electrolyte included in the substrate 10 should be as small as possible so as not to inhibit the permeation of gas components, as long as an electrochemical reaction can occur at the interface between the electrode and the solid electrolyte, and a small amount is sufficient. . The substrate 10 is provided on a frame-shaped support 60 made of plastic or the like. By providing the substrate 10 on the support 60, gas components can enter the substrate IO from the bottom surface of the substrate 10. Adjacent to one side of the substrate 10, an elongated groove-shaped water tank 70 is provided. The water tank 70 is made of plastic or the like. In the illustrated embodiment, one wall surface of the water storage tank 70 is also used as the support stand 60. Water 80 is stored inside the water tank 70. One side of the substrate 10 is bent downward at a right angle, and the tip 11 is inserted into water 80 in a water tank 70.

The sensor action of the gas sensor having the above structure will be explained. As shown by arrows in FIG. 1, the gas components permeate through the substrate 10 from the surface of the substrate 10, reach the working electrode 20, and cause an electrochemical reaction. At the counter electrode 30, a reaction opposite to the above reaction occurs. As a result, the working electrode 20 and the counter electrode 3
A detection current flows during the zero period, and gas components can be detected and quantified. The reference electrode 40 serves as a reference for maintaining the potential of the working electrode 20 constant. The solid electrolyte Jii 50 performs ion conduction between the working electrode 20 and the counter electrode 30. That is, as described above, since the solid electrolyte contained in the substrate 10 is small,
With this solid electrolyte alone, the impedance between the electrodes is extremely high, and ions generated by electrochemical reactions at the electrodes cannot be conducted. However, if the negative side of the electrodes 20 is covered with a thick solid electrolyte layer 50, good ion conduction can be achieved through this solid electrolyte layer 50.

Note that the solid electrolyte layer 50 has a thickness of 30. It is formed to have a thickness of n or more, and there is almost no gas permeation from the surface side of the solid electrolyte layer 50 to the working electrode 20.

The substrate 10 made of a porous body containing a solid electrolyte sucks up water 80 from the water storage tank 70 from one end to keep the solid electrolyte in the substrate 10 at a constant water content, and also absorbs water 80 from the contact portion between the substrate IO and the solid electrolyte layer 50. Moisture is supplied into the solid electrolyte layer 50 to keep the water content of the solid electrolyte layer 50 constant.

Next, specific examples of the electrochemical gas sensor having the above structure will be described in detail.

Example 1 A cellulose filter with a pore size of 0.2 pm was used as the material for the substrate IO. This cellulose filter was fixed on a glass plate, and a perfluorosulfonate polymer solution was cast onto it. The casting amount is 10.5% of the solution containing perfluorosulfonate polymer flow weight % per cj of cellulose filter. crj
! Using. After air drying this for 24 hours, it was peeled off from the glass plate and cut into a predetermined element size.

A working electrode 20° and a counter electrode 30 made of platinum and a reference electrode 40 made of gold were formed on the surface of the substrate 10 thus manufactured by sputtering. These electrodes 2
0... by casting the same perfluorosulfonate polymer solution as above to a thickness of 30tt-
A solid electrolyte layer 50 was formed. This was fixed on an acrylic support 60 and assembled so that one end of the substrate IO made of a cellulose filter was immersed in water 80 in a water tank 70.

In order to confirm the gas sensor function of the sensor element manufactured in this manner, the gas sensitivity to CO gas was measured. Figure 3 shows the test equipment. The sensor element was placed in a measurement chamber 90, and the terminal portions 22 of each electrode 20 were connected to a potentiometer stand 92 with a lead wire 9I. Using the above-mentioned testing device in which the recorder 93 is connected to the potentiostat 92, the working electrode 2
The applied voltage between o and reference electrode 40 was maintained at 0.45V.

The atmosphere inside the chamber 90 changes from air only to CO2.
The current flowing between the working electrode 20 and the counter electrode 3o was measured when the air was replaced with air containing 11000 pp of gas. For comparison, apart from the sensor element of the embodiment according to the present invention, an electrode formed on a silicon substrate with a thickness of 3
A sensor element with a conventional structure, simply covered with a solid electrolyte layer consisting of a μ layer of perfluorosulfonate polymer, was also fabricated and similar measurements were performed.

The measurement results are shown in FIG. 4, and in the graph, the change in output current indicates the sensor sensitivity to CO gas. As is clear from the graph, it can be seen that the example of the present invention has a much higher sensor sensitivity than the comparative example.

Next, in order to evaluate the humidity dependence of sensor sensitivity, similar measurements were performed with various environmental humidity changes. The results are shown in FIG.

Looking at the results, it can be seen that in the case of the comparative example, there is a large variation in the output current due to the difference in humidity, whereas in the example of the present invention, there is almost no variation in the output current. Therefore, according to the present invention, it has been demonstrated that a gas sensor element with extremely low humidity dependence and stable performance can be provided.

Example 2 A sensor element was manufactured through the same steps as in Example 1 except that glass cloth was used as the material for the base ttlo.

The humidity dependence of gas sensitivity was also measured in the same way as above for the sensor element manufactured in this manner. The results are shown in Figure 5, and it was demonstrated that although Example 2 was slightly inferior to Example 1, it was much less dependent on humidity and could exhibit stable performance compared to Comparative Example. Ta.

[Effects of the Invention] According to the electrochemical gas sensor element according to the present invention as described above, one end of the substrate made of a porous body is in contact with water, so water is sucked into the substrate and the solid state of the entire substrate is In addition to keeping the water content of the electrolyte constant, water can also be supplied to the solid electrolyte layer through the contact portion with the substrate, so that the water content of the solid electrolyte layer can be kept constant. As a result, the humidity dependence of the gas sensor element is eliminated, the sensor always has a constant sensitivity regardless of changes in the external environment, and stable performance can be achieved.

Since there is no need to correct the output current using an electric circuit or the like in order to eliminate humidity dependence, the manufacturing cost of the entire sensor is reduced.

In addition, gas components pass through the substrate surface from the surface of the substrate, which is made of a porous material containing a solid electrolyte, and reach the electrodes, and the solid electrolyte layer only needs to perform ionic conduction, so the performance of the gas reaching the electrodes is improved. As a result, the solid electrolyte layer can be made thicker so that ion conduction can be carried out favorably without hindering the gas sensitivity of the sensor element.

[Brief explanation of drawings]

Fig. 1 is a sectional view showing an embodiment of the present invention, Fig. 2 is a plan view, Fig. 3 is a schematic configuration diagram of a sensitivity measuring device, Fig. 4 is a graph showing sensitivity measurement results, and Fig. 5 is a humidity FIG. 3 is a graph diagram showing measurement results of a dependence evaluation test.

Claims (1)

[Claims] 1 A solid electrolyte is contained inside a substrate made of a porous body,
An electrochemical gas sensor element in which a plurality of electrodes are provided on the same surface of a substrate, a solid electrolyte layer is provided covering each electrode and between the electrodes, and a portion of the substrate is brought into contact with water.